Method of seismic surveying



Filed Sept. 21, 1943 Conqmi RHCHERT Patented Sept. 18, 1945 METHOD OF SEISMIC SURVEYING Conrad Beichert, San Antonio, Tex., assignor to Olive S. Petty, San Antonio, Tex.

Application September 21, 1943, Serial No. 503,261

1 Claim.

This invention relates to improvements in methods of propagating seismic impulses by the detonation beneath the ground of an explosive charge, a practice common in the field of seismic surveying.

It is customary in reflecting shooting to detonate the explosive charge at the bottom of a drill hole or shot hole of considerabledepth, sometimes as much as five hundred feet, the depth depending upon local conditions and being in general sumcient to insure that the explosion will occur in one of the more solid geological structures below the so-called weathered layer of the earth. However. even when the charge is exploded at considerable depth, a substantial quantity of solid material is usually blown out of the upper end of the hole by the explosion, and when. as is often the case, this material falls toward the detecting stations at which the seismic energy reflected to the surface is received for transmission to suitable recording apparatus; surface vibrations are thereby set up which arrive at the detecting stations during the arrival of the reflected energy, so that the record of the latter is rendered confusing and is difficult to interpret.

It has heretofore been proposed, for example in the patent to Dudley and Lucid, 1,943,725, granted January 16, 1934, to pack the shot hole with dirt for its entire depth above the char e in order to confine the explosive force of the charge so that the effectiveness of the explosive will be concentrated, and this procedure when properly followed also prevents the scattering of solid material from the upper end of the hole, or sufliciently delays the ejection of solid material so that the reflected waves are received and recorded before disturbances created by the falling of solid material reach the detecting points. However, this method renders difficult the subsequent use of the same hole, for the firing of further charges, although this is frequently a very desirable practice, since the packed earth must be removed before a fresh charge can be introduced.

I have fourid that the desired result of preventing or delaying to a sufficient extent the ejection of the solid material from the shot hole can be accomplished by packing the upper end of the hole only to the relatively small depth of a few feet,

and that by this expedient the repeated use of the same hole for the firing of successive charges is not interfered with, as in the case where the entire hole is packed.

It is therefore an object of the instant invention to provide a method of propagating seismic waves in surface surveys which includes the step of obstructing the upper end of the shot hole in which the charge is fired to a depth only sulficient to insure delay of the ejection of solid material for a period of time required to avoid interference with the recorded reflected energy.

Further objects and features of the invention will be apparent from the followingdescription taken in connection with the accompanying drawing in which the figure represents a vertical sectional view through a drill hole or shot holeillustrating the application of the present method in the propagation of seismic impulses.

Referring specifically to the drawing, it will be observed that the shot hole i0, is shown as extending downwardly through a weathered layer I I and; through various beds or geological formations. l2, l3, l4, and It. The usual solid explosive charge and blasting cap 20' is disposed at the bottom of the hole and the conducting leads H by means of which the charge is electrically fired extend from the cap upwardly through the hole and out at the surface to asuitable blasting machine (not shown) by means-of'which the conductors are energized and the 'cap is detonated in a manner well known in the art. The shot hole may be filled to a considerable depth above the cap with water, for example to a level indicated at 24, for the purpose of confining the exploslon to a limited extent, although this is optional.

In the preferred practice of my'invention I introduce in the upper end of the shot hole for a distance of a few feet, for example three or four feet or more, an obstruction 25. This obstruction may be constituted by a metal can, a wooden plug or any other member which may conveniently be forced into the upper end of the shot hole and which is so dimensioned as to fit the hole snugly and to require substantial pressure to force it further down the hole; The hole above the obstruction is then filled with dirt or sand asindicated at 26, preferably tamped firmly in position.

The location of the obstruction 25 at a depth of a few feet is usually sufllcient to delay the ejection of material from the hole by the force of the explosion so that disturbances created in the surface layer are delayed and the recording of the reflected energy can be effected without interference. Packing of the hole to notsubstantially greater depths will often prevent entirely the ejection of solid material depending upon the depth of the hole, the size of the explosive charge. and other factors. After the firing of the charge, any of the packing 25 which-may remain in the hole can readily be removed, by an auger or other suitable tool, so that a subsequent charge may be introduced in the same hole with a minimum of en'ort and delay. If desired, a rope or the like may be secured to the obstruction 25 and made last on the surface so as to prevent unintentional displacement of the obstruction oi the hole.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

A method of propagating seismic waves in subsurface surveys which includes the steps of locating a solid explosive charge and a blasting cap in a shot hole oi. substantial depth, plugging the hole adjacent the upper end only. so as to leave the major portion of the hole above the charge unobstructed, and firing the cap, whereby ejection of solid material from the hole by the exploding charge is delayed for a. period which is at least sufllcient to permit the reception and recording of the. significant seismic impulses prior to the reception of disturbances resulting from such ejection, and whereby the hole may readily be cleared for the loading therein of another explosive charge.

CONRAD REICHIRT. 

